This disclosure relates generally to light modulation and in particular to a light modulator using total internal reflection at an interface with a tunable conductive layer.
Terahertz radiation generally refers to electromagnetic radiation (light) having a frequency in a range from about 0.1 terahertz (THz) to about 300 THz (wavelength in a range from about 3 mm to about 1 μm), including mid- to far-infrared light. Such radiation has potential application in a number of fields, including high-bandwidth communication and imaging (e.g., for medical diagnostics and other applications in biology).
However, optical components to modulate the phase and/or intensity of terahertz radiation are not well developed, particularly for broadband applications. Techniques used in other frequency ranges (e.g., optical frequencies) are generally not suitable for terahertz radiation. Thus, new structures and techniques are desired.
Various modulators for terahertz radiation have been proposed. For example, in regard to intensity modulation, electrically controllable metamaterial structures based on split ring resonators (SRRs) have been fabricated on gallium arsenide (GaAs) semiconductor substrates. (H.-T. Chen et al., “Active terahertz metamaterial devices,” Nature vol. 444, no. 7119, pp. 597-600 (November 2006); H.-T. Chen et al., “Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics vol. 2, no. 5, pp. 295-298 (April 2008)). Such structures can modulate transmission of a terahertz pulse at a specific frequency. However, the frequency range is limited, and the transmission loss is generally high (e.g., 50% loss with the device in its off state). As another example, graphene sandwiched between a hexagonal grid and a pair of wire grids has been used to modulate the transmission for a resonance using a high voltage (e.g., 600 V). (S. H. Lee et al., “Switching terahertz waves with gate-controlled active graphene metamaterials,” Nat. Mater. vol. 11, no. 11, pp. 936-941 (2012)). As yet another example, SRRs fabricated on a graphene layer have provided 18% modulation depth with lower gate voltages, but only at a specific frequency. (R. D. Innocenti et al., “Low-Bias Terahertz Amplitude Modulator Based on Split-Ring Resonators and Graphene,” ACS Nano vol. 8, no. 3, pp. 2548-2554 (2014)). In another example, patterning a graphene layer with a set of ring apertures has been used to increase the modulation depth to around 50% over a small range of frequencies. (W. Gao et al, “High-contrast terahertz wave modulation by gated graphene enhanced by extraordinary transmission through ring apertures,” Nano Lett. vol. 14, no. 3, pp. 1242-1248 (2014)). Still another example employs graphene deposited on top of a linear polarizer structure, providing 80% modulation depth over a larger frequency range with relatively low gate voltages. (S. F. Shi et al., “Optimizing Broadband Terahertz Modulation with Hybrid Graphene/Metasurface Structures,” Nano Lett. vol. 15, pp. 372-377 (2015)). However, the grating parameters of the linear polarizer limit the frequency range, and performance rapidly falls off as frequency increases.
Examples of phase modulators include a modulator that uses the resonance of an electric SRR array and an external voltage to alter the phase of a terahertz pulse at a single frequency (just off the resonant peak of the SRR array). (H. Chen et al., “A metamaterial sold-state terahertz phase modulator,” Nat. Photonics vol. 3, pp. 148-151 (2009)). Gate-controlled graphene metasurfaces provide a controllable phase modulation but with variable intensity in the on and off states. (Z. Miao et al., “Widely Tunable Terahertz Phase Modulation, with Gate-Controlled Graphene Materials,” Phys. Rev. X, vol. 5 p. 014027 (2015)).
In all of the above examples, the frequency range is limited, in some cases to a very small range. This is in part due to the reliance on resonance phenomena. Some of the above examples also suffer from other drawbacks, such as high voltages and/or inefficient transmission of light when the modulator is in its off state.
Another approach uses total internal reflection (TIR) at a surface with a variable index of refraction to provide spatial modulation in the terahertz range. (M. Koch et al., “Modulator of electromagnetic waves,” European Patent Publication 2 597 509 A1 (2013)). For example, a liquid crystal (LC) cell can be placed on the TIR surface of a prism, and the refractive index of the LC cell can be controlled (e.g., by applying a voltage to change the orientation of the LC molecules) to allow switching between TIR and non-TIR conditions. However, relatively thick LC cells would be required, and the voltage needed to switch such cells may be expected to result in slow and inefficient operation.
Accordingly, improved modulators for terahertz radiation would be desirable. Such modulators may provide, among other things, improved efficiency, broadband operating capability, good modulation depth, and/or other desirable features.